This invention relates to a draw twisting machine provided with a plurality of synchronous motors which drive fiber (thread, strand, yarn, filament or the like) draw rollers. The invention relates more particularly to a draw twisting machine which is provided with a plurality of synchronous motors arranged to drive respective draw rollers of the machine. The draw rollers should run at substantially identical angular velocities (rpm) and be able to be stopped at substantially the same angular position. The electrical supply for the synchronous motors is provided by a variable frequency A.C. generator driven by a D.C. motor, the output frequency of the A.C. generator being proportional to the angular velocity (rpm) of a shaft which couples the D.C. motor to the A.C. generator. The angular velocity of the D.C. motor may be varied in a stepless manner.
The high fiber transport velocities which occur in fast running draw twisting machines require that the draw rollers, which pull the fibers from stretch or shrinking zones, be driven by individual electric motors, because a common drive of the draw rollers by bevel gears or the like does not meet the requirements of the high angular velocity of the draw rollers. On the other hand, a single delivery roller effecting the delivery of the fibers into the stretching or shrinking zones of the draw twisting machine can be used to deliver fibers to all stretching or shrinking zones of one side of the machine, so that, normally, only two delivery rollers are necessary (one for each side of the machine). These two delivery rollers are also driven by synchronous motors. With respect to draw rollers and with respect to the delivery rollers there is a requirement, in practice, that, first of all, all the draw rollers should run as closely as possible in angular synchronism with respect to one another and a requirement that, furthermore, these rollers should do so with respect to the delivery rollers. If these requirements are not met, the stretching or shrinking zones would non-uniformly stretch or shrink the fibers running therethrough. The drive motors for the draw rollers and for the delivery rollers are, for this reason, synchronous motors which are supplied, in a known manner, from a common, variable frequency A.C. generator which can be disposed either on the machine or external to the machine and which is itself driven by a D.C. motor whose velocity (rpm) is variable and can be regulated.
In known drive assemblies of the above-mentioned type, the startup of the synchronous motors is effected by firstly driving the variable frequency A.C. generator by the D.C. motor at a crawl speed of, for example, 40 rpm which generates an output frequency from the A.C. generator of 1 Hz, for example, and subsequently, the synchronous motors are switched on simultaneously while the A.C. generator provides this crawl-speed frequency.
Because of the speed sensitive characteristics of D.C. motors at low speeds, a load impulse occurs during switch on, and the speed of the D.C. motor and, therefore, that of the variable frequency A.C. generator begins to oscillate. This leads undesirably to oscillation of the draw rollers and subsequently to the looping of fibers or to their falling off from the draw rollers or to their breakage. The speed regulation of the D.C. motor is too slow to prevent the unwanted oscillation and the undesirable consequences. If one were to attempt to prevent the undesirable oscillation by associating a large inertial mass with the D.C. motor or with the variable frequency A.C. generator, the acceleration and the deceleration (braking) of the variable frequency A.C. generator would be undesirably retarded.